Aluminum nitride (AlN) is expected to be a material for ultraviolet light emitting device since it has as great as 6.2 eV of the forbidden band gap and it is a directly transmitting-type semiconductor.
In order to form a semiconductor device including a light emitting device, it is necessary to form a n-type semiconductor layer jointed electrically to a n-electrode, a p-type semiconductor layer jointed electrically to a p-electrode and a laminated structure therebetween, where in the laminated structure comprises a clad layer, an active layer and the like. For the semiconductor device, it is required in view of light emitting efficiency that each of those layers consist of a crystal having high crystallinity, that is, having low density of point defect or dislocation and that the difference of the lattice constant and the difference of thermal expansion coefficient between the crystal and aluminum gallium nitride (AlGaN) which forms the laminated structure are small, and it is also required in view of preventing degradation of the device that the crystal has high heat conductivity. Therefore, it is advantageous for making an AlN semiconductor device to form the above layered structure on an AlN single crystal substrate.
The AlN single crystal substrate is usually produced by the vapor-phase growing method in which the AlN single crystal is grown on a single crystal substrate such as sapphire since it is difficult to make the homoepitaxial substrate. Among the vapor-phase growing methods, the hydride vapor phase epitaxy (HVPE) is known as the method which has the high deposition rate. The HVPE method is not suited for forming the crystal layer of the semiconductor light emitting device because it is relatively difficult to precisely control the film thickness compared to the molecular beam epitaxy (MBE) method and the metal organic vapor phase epitaxy (MOVPE) method, though it is possible to obtain the single crystal of good crystallinity at a high deposition rate. Therefor, it is also possible to manufacture the substrate consisted of only the AlN single crystal (AlN-selfsupporting substrate) by forming a thick film of the AlN single crystal layer on a substrate such as sapphire using the HVPE method followed by separating it (see JP-A 2005-252248).
When manufacturing the AlN semiconductor device, forming of the n-type conductive aluminum nitride semiconductor crystal layer on a substrate is required first, and as a method of forming the n-type conductive aluminum nitride semiconductor crystal, the approach of forming the n-type conductive AlN crystal layer wherein being doped with Si by the MOVPE method is known (see JP-A 2003-273398). In addition, as for the InGaN, AlGaN, and AlInGaN crystal systems, the method of forming the n-type conductive crystal layer wherein being doped with Si by the HVPE method is known (JP-A 2000-91234).